You will not get on a small fleet as a DEP so you have several years of initial freeze to read the rules (!) but:
Reserve blocks are shorter than the 28 days on mainline
You can bid for reserve and state preferences for trips
There is no 'bank'
There will be an agreed number of 'trump' days - to be used to get specific days off etc. These are handled in seniority order.
All known work will be allocated leaving very little uncovered at roster publication.
In summary, very different from mainline, would not suit everyone.
It is unlikely that BA will recruit DEPs onto a 'small fleet' now, given that we are talking about the 380 and 787. By the time you get into a position to be on such fleets, it will probably all have changed, certainly on the 787, but the 380 will remain 'small fleet'.